Why are the cells generally of a small size?

Cells are generally so little so that they can maximize their ratio of area to volume. They have to be ready to get the nutrients in and therefore the waste out quickly. The important point is that the area to the volume ratio gets smaller as the cell gets larger. Thus, if the cell grows beyond a particular limit, not enough material is going to be ready to cross the membrane fast enough to accommodate the increased cellular volume. That's why cells are so small.